Geochimica et Cosmochimica Acta, Vol. 68, No. 4, pp. 841–865, 2004 Copyright © 2004 Elsevier Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/04 $30.00 ϩ .00 doi:10.1016/j.gca.2003.07.009 Volcanic arc of Kamchatka: a province with high-␦18O magma sources and large-scale 18O/16O depletion of the upper crust 1, 2 3 1 ILYA N. BINDEMAN, *VERA V. PONOMAREVA, JOHN C. BAILEY, and JOHN W. VALLEY 1Department of Geology and Geophysics, University of Wisconsin, Madison, WI, USA 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk-Kamchatsky, Russia 3Geologisk Institut, University of Copenhagen, Copenhagen, Denmark (Received March 20, 2003; accepted in revised form July 16, 2003) Abstract—We present the results of a regional study of oxygen and Sr-Nd-Pb isotopes of Pleistocene to Recent arc volcanism in the Kamchatka Peninsula and the Kuriles, with emphasis on the largest caldera- forming centers. The ␦18O values of phenocrysts, in combination with numerical crystallization modeling (MELTS) and experimental fractionation factors, are used to derive best estimates of primary values for ␦18O(magma). Magmatic ␦18O values span 3.5‰ and are correlated with whole-rock Sr-Nd-Pb isotopes and major elements. Our data show that Kamchatka is a region of isotopic diversity with high-␦18O basaltic magmas (sampling mantle to lower crustal high-␦18O sources), and low-␦18O silicic volcanism (sampling low-␦18O upper crust). Among one hundred Holocene and Late Pleistocene eruptive units from 23 volcanic centers, one half represents low-␦18O magmas (ϩ4 to 5‰). Most low-␦ 18O magmas are voluminous silicic ignimbrites related to large Ͼ10 km3 caldera-forming eruptions and subsequent intracaldera lavas and domes: Holocene multi-caldera Ksudach volcano, Karymsky and Kurile Lake-Iliinsky calderas, and Late Pleistocene Maly Semyachik, Akademy Nauk, and Uzon calderas. Low-␦18O magmas are not found among the less voluminous products of stratovolcano eruptions and these volcanoes do not show drastic changes in ␦18O during their evolution. Additionally, high-␦18O(magma) of ϩ6.0 to 7.5‰ are found among basalts and basaltic andesites of Bezymianny, Shiveluch, Avachinsky, and Koryaksky volcanoes, and dacites and rhyolites of Opala and Khangar volcanoes (7.1–8.0‰). Phenocrysts in volcanic rocks from the adjacent Kurile Islands (ignimbrites and lavas) define normal-␦18O magmas. The widespread and volumetric abundance of low-␦18O magmas in the large landmass of Kamchatka is possibly related to a combination of near-surface volcanic processes, the effects of the last glaciation on high-latitude meteoric waters, and extensive geyser and hydrothermal systems that are matched only by Iceland. Sr and Pb isotopic compositions of normal and low-␦18O, predominantly silicic, volcanic rocks show negative correlation with ␦18O, similar to the trend in Iceland. This indicates that low-␦18O volcanic rocks are largely produced by remelting of older, more radiogenic, hydrothermally altered crust that suffered ␦18O-depletion during Ͼ2 My-long Pleistocene glaci- ation. The regionally-distributed high-␦18O values for basic volcanism (ca. ϩ 6toϩ7.5‰) in Kamchatka cannot be solely explained by high-␦18O slab fluid or melt (Ϯ sediment) addition in the mantle, or local subduction of hydrated OIB-type crust of the Hawaii-Emperor chain. Overall, Nd-Pb isotope systematics are MORB-like. Voluminous basic volcanism (in the Central Kamchatka Depression in particular) requires regional, though perhaps patchy, remobilization of thick (30–45 km) Mesozoic-Miocene arc roots, possibly resulting from interaction with hot (ca. 1300°C), wedge-derived normal-␦18O, low-87Sr/86Sr basalts and from dehydration melting of lower crustal metabasalts, variably high in ␦18O and 87Sr/86Sr. Copyright © 2004 Elsevier Ltd 1. INTRODUCTION ice core record are now attributed to sources in Kamchatka and the Aleutian island arc (e.g., Zielinski et al., 1996; Braitseva et Dynamic interplay between glaciations and volcanic activity is seen during the Pleistocene history of the northern circum- al., 1997b). The Kamchatka Peninsula overlies the northwest- Pacific region. Voluminous caldera-forming eruptions, glacial ern margin of the subducting Pacific plate and is one of the advances, and rapid sea level changes, have affected local and most volcanically active area in the Holocene (Simkin and global climate; recent studies recognize the importance of the Siebert, 1994), and probably has the largest exposed hydrother- North Pacific in paleoclimate reconstruction (e.g., Bradley, mal and geyser system at high latitudes. 2000; Gualtieri et al., 2001; Beget, 2001; Brigham-Grette, This paper was prompted by the discovery that Kamchatka is 2001). Glaciation may have local and regional effects on vol- a land of isotopic diversity with high-␦18O basic magmas and canism; rapid pressure release during deglaciation may help low-␦18O upper crust. Low-␦18O magmas are here defined as trigger eruptions (e.g., Nakada and Yokose, 1992), but the Ͻ5.5‰ for basalts to Ͻ 5.8‰ for rhyolites; high-␦18O magmas broader effects are poorly understood. Many ash layers and are Ͼ6.0‰ for basalts, and Ͼ6.4‰ for rhyolites, see section H2SO4-rich horizons in the Pleistocene-Holocene Greenland 4.1 below. Evidence is presented that Kamchatka is a high- latitude volcanic province with an abundance of low-␦18O rocks (based on number of low-␦18O units and their total * Author to whom correspondence should be addressed, at Division of Geological and Planetary Sciences, California Institute of Technology, volume) matched only by Iceland. We attribute this to the effect Pasadena, CA 91125, USA ([email protected]). of Pleistocene glaciation on meteoric waters, hydrothermally- 841 842 I. N. Bindeman et al. altered rocks, and magmas (cf. Donnely-Nolan, 1998; Gautason tephras create a framework of Holocene volcanic activity, and Muehlenbachs, 1998; Bindeman et al., 2001). which is pertinent to volcanology, dating marine sediments No low-␦18O magmas have been found in low-latitude arcs (Prueher and Rea, 2001a), impact on climate (Braitseva et al., of the circum-Pacific, or other island arcs, but normal- and 1997b; Prueher and Rea, 2001b), and archeology (Braitseva et high-␦18O magmas are common (Taylor, 1968; Matsuhisa, al., 1987). Pleistocene and older ignimbrite and tephra layers 1979; Macpherson et al., 1998; Vroon et al., 2001). At high- are unambiguously found in the vicinity of large calderas (Ͼ10 latitudes, the Fisher and Okmok calderas (Aleutians) produced km in diameter): Pauzhetka, Uzon, Maly Semyachik, Akademy low-␦18O magmas (Bindeman et al., 2001), and more low-␦18O Nauk, and others (e.g., Grib and Leonov, 1993a,b). Most te- magmas are likely to be found there: like Kamchatka, the phras at each eruptive center represent differentiated products, Aleutian arc is a volcanically active subpolar area that under- typically silicic andesite to rhyolite, occurring in caldera set- went Pleistocene glaciation. tings, and are distinct mineralogically and chemically. Given This study uses the primary range of magmatic ␦18O values the predominantly NE direction of winds, tephra layers from in Kamchatkan magmas based on analyses of phenocrysts in major eruptions have been deposited in the Beringia Terrain combination with the radiogenic isotopes of Sr, Nd, and Pb (Fig. 1, inset), and numerous layers of abundant Kamchatkan determined on whole rocks to provide key constraints on the volcanic ash are identified in ODP marine sediments in the NW genesis of both high- and low-␦18O magmas discovered in Pacific (Bailey, 1993, 1996; Prueher and Rea, 2001a,b). Kamchatka. Such an approach is used to define the role of deep (slab and mantle) processes vs. shallow processes of crustal 2.2. Glaciations recycling, and was used in the past to describe more subdued ␦18O ranges in other island arcs and other settings (e.g., Thirl- Kamchatka experienced several voluminous glaciations dur- wall et al., 1996; Baker et al., 2000; Eiler et al., 2000a,b; Vroon ing Pleistocene (Braitseva et al., 1968; Grosswald, 1998). Ice et al., 2001). We here present evidence that lower and upper covered most volcanic edifices even in southern Kamchatka crustal processes dominate the petrogenesis of basic and silicic (Melekestsev et al., 1974; Savoskul, 1999). Starting from the magmas in Kamchatka, though a mantle signature can still be beginning of glaciation at 2.6 Ma, ice repeatedly reached sea- recognized in some trace elements and isotopes. shores and supplied debris and iceberg-rafted material to ma- rine sediments (Prueher and Rea, 2001b). At present, the snow ϳ 2. KAMCHATKAN VOLCANISM line is at 1000m elevation, and many volcanic edifices con- tain patchy glaciers. 2.1. Eruptive Centers, Volcanic Rocks, and 14C-Dated Tephra Layers 2.3. Previous Stable Isotope Studies The Kamchatka volcanic belt is a part of the Kurile-Kam- Phenocryst-based oxygen isotope studies in Kamchatka have chatka volcanic arc related to the subduction of the Pacific plate been limited; most reported analyses are for whole-rocks that (Fig. 1). Eruptive centers differ in their tectonic position rela- are prone to postmagmatic alteration, and concentrated on the tive to the present volcanic front and can be grouped into three petrogenesis of basalts. Pokrovsky and Volynets (1999) and tectonic zones (Fig. 1): Eastern Volcanic Front (EVF), Central Pineau et al. (1999) demonstrated a moderate (ϳ1‰) increase Kamchatka Depression (CKD), and Sredinny Range (SR). in whole-rock ␦18O values across the arc. Dorendorf et al. Magmatic volumes generally decrease westward, but Recent (2000) discovered that olivines and pyroxenes from Klyuchev- volcanism is most voluminous in the CKD with some of the skoy volcano have high-␦18O values (6–7.5‰) and attributed largest volcanoes of the world such as Klyuchevskoy. Volcanic this to the addition of high-␦18O slab fluids to the mantle rocks of the Kurile-Kamchatka island arc span compositions generation zones. In the study of differentiated products, Pok- from basalt to rhyolite, from tholeiitic to calc-alkaline, and rovsky and Volynets (1999) noted that many ignimbrites in from low- to high-K and shoshonitic series (Fig.